A high voltage direct current (HVDC) converter converts electric power from high voltage alternating current (AC) to high voltage direct current, or vice-versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies. A complete converter station can contain several such converters in series and/or parallel (multi-module HVDC converters).
HVDC converters are divided into two main categories: line-commutated converters and forced-commutated voltage-sourced converters. Line-commutated converters use on-controlled and off-uncontrolled switches; i.e., thyristors. Forced-commutated voltage-source converters (VSCs) use on-controlled and off-uncontrolled devices, namely insulated-gate bipolar transistors (IGBTs).
At present, both the line-commutated and voltage-source technologies are important, with line-commutated converters used mainly where very high capacity and efficiency are needed, and voltage-source converters used mainly for interconnecting weak AC systems, such as connecting large-scale wind power to the grid. The market for HVDC voltage-source converters is growing relatively fast, driven partly by the surge in investment in offshore wind power, with one particular type of converter, the Modular Multi-Level Converter (MMC) emerging as a front-runner, due to its modularity and scalability.
Like the two-level converter and the six-pulse line-commutated converter, a MMC typically can consist of six valves (or “arms”), each connecting one AC terminal to one DC terminal. However, where each valve of the two-level converter can be effectively a high voltage controlled switch consisting of a large number of IGBTs connected in series, each arm of a MMC can be a separate controllable voltage source in its own right. Generally, each MMC arm consists of a number of independent converter sub-modules, each containing its own storage capacitor. In the most common form of the circuit, the half-bridge variant, each sub-module contains two IGBTs connected in series across the capacitor, with the midpoint connection and one of the two capacitor terminals brought out as external connections. Depending on which of the two IGBTs in each sub-module is turned on, the capacitor is either bypassed or connected into the circuit. Each sub-module therefore can act as an independent two-level converter generating a voltage of either 0 or Vsm (where Vsmis the sub-module capacitor voltage). With a suitable number of sub-modules connected in series, the valve can synthesize a stepped voltage waveform that approximates very closely to a sine-wave and typically contains very low levels of harmonic distortion. The MMC differs from other types of converters in that current typically flows continuously in all six arms of the converter throughout the mains-frequency cycle. As a result, concepts such as “on-state” and “off-state” have no meaning in the MMC. The direct current splits equally into the converter legs and the alternating current splits equally into the upper and lower arm of each phase.
In HVDC systems, limiting fault currents is vital to protect the converter semiconductor devices, which are the most sensitive components in the system. Unfortunately, the VSC and half-bridge MMC are defenseless against DC side faults since their free-wheeling diodes function as an uncontrolled rectifier bridge and feed the DC fault, even if the semiconductor devices are turned off. During the DC fault, the AC side current contribution into DC fault passes through the free-wheeling diodes. As a result, the diodes can be damaged due to the high fault current. This rectification mode of operation is shown in FIGS. 2A and 2B for the two-level VSC and half-bridge MMC, respectively, during a DC side fault.
In FIG. 2A, the DC fault current (iF) in VSC-HVDC systems emanates from the contribution of both the AC grid (igc) along with the discharging current of the DC-link capacitor (idis). The discharging current has a large first peak that decays with time. In a MMC-HVDC system, the common DC-link capacitor is not utilized, which helps suppress the discharge current. However, the fault current contribution from the AC side (igc) still exists; i.e., there is no segregation between the DC and AC sides during DC faults in this topology. A solid state DC circuit breaker can be used to overcome the DC side problems in HVDC converters, however its main drawbacks are generally cost and relatively high conduction losses.
AC circuit breakers (ACCBs) can be used to achieve the required DC protection, but the free-wheeling diodes used with IGBTs are fast recovery diodes characterized by low surge current withstand capability. These free-wheeling diodes should withstand the fault current until the circuit breaker trips. Thus, there can be a risk in depending on ACCB protection alone, since the semiconductor devices can be damaged due to high fault currents. To enhance the reliability of AC circuit breakers in DC protection, converter embedded devices can be used in conjunction with the AC circuit breakers.
In the Single Thyristor Switch Scheme (STSS), a single thyristor switch is connected in each sub-module of the MMC, as shown in FIG. 3A. The thyristor is used to share the fault current with a free-wheeling diode or, in the other words, to reduce the overcurrent stresses on semiconductor devices pending the tripping of the ACCBs. This can be realized by turning the thyristors on when a DC side fault is detected. The thyristor has a higher capability for withstanding the surge current compared to the free-wheeling diode. As a result, most of the fault current flows through the thyristor and not through the diode.
Because the STSS protects the semiconductor devices but typically cannot prevent the grid current contribution into the DC fault, an evolution was later introduced to address this shortcoming. The Double Thyristor Switch Scheme (DTSS) can be used to protect the semiconductor devices by sharing the current with the free-wheeling diodes and simultaneously prevent the grid current contribution, which can allow the DC-link current to freely decay. In this scheme, a double thyristor switch (back-to-back thyristor) is connected across the semiconductor devices, as shown in FIG. 3B. However, the main drawbacks of this method generally are: both thyristors have to withstand high dv/dt during normal operation (high dv/dt can produce capacitive displacement current in the device, which can cause undesirable turn on); a snubber circuit is also typically essential to prevent damage due to overvoltage spikes and dv/dt; the free-wheeling diodes are still sharing the fault current with the thyristors; and the complexity of implementing an embedded double thyristor switch in each MMC sub-module.
The STSS and DTSS can also be applied to the VSC configuration by connecting the thyristor across each semiconductor device, as shown in FIGS. 3C and 3D, respectively. Given the above drawbacks, it would be desirable to provide a protection scheme for HVDC converters (classical two-level VSC as well as half-bridge MMC) in which, instead of connecting the double thyristor switches across the semiconductor devices, they are combined and connected across the AC output terminals of the HVDC converter.
Thus, a DC side fault isolator for HVDC converters solving the aforementioned problems is desired.